1. Field of the Invention
The present invention relates generally to piezoelectric electro-acoustic transducers adaptable for use as piezoelectric buzzers and the like, and more particularly, to piezoelectric electro-acoustic transducers including a piezoelectric diaphragm having an improved structure for lowering the resonant frequency of the transducer.
2. Description of the Prior Art
A conventional piezoelectric electro-acoustic transducer is disclosed in, for example, Published Unexamined Japanese Patent Application No. 52-24399. This prior art device includes a piezoelectric diaphragm which is supported by a first cylindrical casing having a relatively greater diameter and a second cylindrical casing having relatively smaller diameter. More specifically, in the prior art device, an "intermediate" step-like section extends circumferentially along the inner wall surface of the first cylindrical casing at a position corresponding to a vertical midpoint thereof, causing the piezoelectric diaphragm to be sandwiched at its peripheral portion between the intermediate step-like section and the terminal edge of the first and second cylindrical casings thereby to provide a rigid support for the piezoelectric diaphragm.
Unfortunately, such a piezoelectric electro-acoustic transducer of the type disclosed in Unexamined Japanese Patent Application No. 52-24399 experiences a problem. Because the piezoelectric diaphragm is supported along its entire circumference at the peripheral edge thereof, if an acoustic or sound pressure, e.g., the resonance frequency, must be shifted or offset into an even lower range, the piezoelectric diaphragm might be increased in diameter or alternatively decreased in thickness.
Obviously, where the piezoelectric diaphragm is enlarged in diameter, the resultant piezoelectric electro-acoustic transducer has a correspondingly increased size. When the piezoelectric diaphragm is made to be thinner, it is required that the piezoelectric ceramic plate constituting the piezoelectric diaphragm and/or a metal plate onto which the piezoelectric ceramic plate is adhered be reduced in thickness, which in turn causes difficulty of manufacture, an increase in cost and/or a decrease in stability of characteristics.
Another prior art piezoelectric electro-acoustic transducer is disclosed in, for example, Published Unexamined Japanese Utility-Model Publication No. 5-90594, which transducer is capable of attaining a peak sound pressure in a much lower frequency range without having to modify the diameter and/or thickness of the piezoelectric diaphragm.
With the piezoelectric electro-acoustic transducer disclosed in Unexamined Japanese Utility-Model Publication No. 5-90594, a disk-shaped piezoelectric diaphragm is supported by a first cylindrical casing and a second cylindrical casing inserted into the first casing. More specifically, the disk-like piezoelectric diaphragm is supported by a combination of an intermediate step-like section circumferentially extending on the inner wall of first cylindrical casing and the opening edge surface of second cylindrical casing.
In order to force the acoustic peak point to shift toward the low-frequency side, this prior art device is provided with cut-away portions formed at selected locations in the first and second cylindrical casings at which the piezoelectric diaphragm is supported. Such cutaway portions permit partial support of the piezoelectric diaphragm only at a part of the circumferential edge along the periphery of piezoelectric diaphragm.
According to the description in Unexamined Japanese Utility-Model Publication No. 5-90594, it has been explained that the low-frequency shift of an acoustic pressure peak can be accomplished by partially supporting the piezoelectric diaphragm at the periphery thereof.
A problem with such devices as shown and described in Unexamined Japanese Utility-Model Publication No. 5-90594 is that the arrangement of the piezoelectric diaphragm relative to the first and second cylindrical casings causes stress to be transmitted from the piezoelectric diaphragm to the first and second cylindrical casings. The formation of the cut-away portions in the first and second cylindrical casings, while lowering the acoustic pressure peak, do not suppress or prevent the transmission of stress from the piezoelectric diaphragm to the first and second cylindrical casings.
Another serious problem encountered with the prior art devices is that they suffer from the existence of limits for such low-frequency of the acoustic pressure even with use of such a partial support structure resulting from an improvement in the casing structure for mechanical support of the disk-like piezoelectric diaphragm by the first and second cylindrical casings. In other words, the resonance frequency can never be simply lowered in value and there might be a limit for any further decrease in resonance frequency beyond a certain value. This makes it impossible or at least extremely difficult to use this prior art structure for some applications which strictly require achievement of extra low resonance frequency.
Another disadvantage of the prior art structure disclosed in Unexamined Japanese Utility-Model Publication No. 5-90594 is that the use of specific casing members of a specially designed shape is required for achievement of such partial support of the piezoelectric diaphragm. This results in an unwanted increase in manufacturing cost because a variety of type of unique and specially designed casing members must be prepared in accordance with a target value of the resonance frequency in a case-by-case manner.